Modern commercial vehicles with pneumatic brakes exhibit, in addition, an electronic braking system which, among other things, has an anti-lock function. An electronic brake control unit (EBS ECU) processes signals of wheel-speed sensors and, where appropriate, intervenes in the actuation of the brakes.
The electronic braking system performs further functions. For this purpose the signals of further sensors may be evaluated. The further sensors may have been integrated into the brake control unit, may have been directly linked to the brake control unit via lines, or signals of further sensors may be communicated to the brake control unit via a data-bus system, for instance a CAN data bus. A lateral-acceleration sensor has preferentially been integrated into the brake control unit. In the case of pneumatically suspended vehicles, a bellows-pressure sensor registers the level of the loading or axle load. In the case of mechanically suspended vehicles, spring-deflection sensors may have been provided for the same purpose.
Trailer vehicles may have been equipped with their own electronic braking system and/or with a brake control unit. The electronic braking system of the trailer vehicle has been connected to the corresponding system of the towing vehicle via a standardized plug-in connection. A data-bus link may have been integrated into the plug-in connection or may be additionally present. As a result, signals acquired in the towing vehicle can be transferred to the trailer vehicle.
For reasons of safety, if no signals are provided by the wheel-speed sensors after expiration of a definite time-interval after switching on the electronic braking system, an error is presumed and an error message is output. For example, the wheel-speed sensors may be defective, may not have been inserted correctly, or cable links may be defective. In fact, an error does not have to be present in every case. For instance, with the electronic braking system switched on and without the parking-brake engaged, quite long standstill-times may arise, for example in the course of a stay in a workshop, or for other reasons.
If a vehicle has lift axles and the latter are equipped with wheel-speed sensors, the case may arise that the vehicle is in motion, the wheel-speed sensors in question are correctly transmitting no signals, on account of the raised axles, and nevertheless an error message is generated, since signals of the wheel-speed sensors are expected within the defined time-interval.